Mojave On Internal HD - Catalina On External SSD

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Level 1

59 points

Mojave On Internal HD - Catalina On External SSD

I am on a 2017 iMac running Mojave 10.14.6 - 16GB RAM

I want to instal Catalina on an external LaCie SSD properly formatted. When I run the installer The external drive is filled with files (see attached photo) and an installer is placed on my internal drive. But the installation is always unsuccessful. When I run the installer in my Applications Folder I end up with the same folder on my SSD and still no OS.

Any advise would be much appreciated.

iMac 27″ 5K, macOS 10.14

Posted on May 24, 2025 01:53 PM

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Answer 1

HWTech

Level 9

66,806 points

May 24, 2025 08:03 PM in response to Steve Cohen

It sounds like you have downloaded the Catalina installer while booted into macOS Mojave on the internal drive and are running that downloaded installer to install Catalina to the external drive?

You will need to erase the physical external drive as GUID partition and APFS (top option) before selecting the external drive as a destination for the full install of macOS. You may need to click "View" within Disk Utility and select "Show All Devices" before the physical drives appear on the left pane of Disk Utility. Of course doing this will destroy the data on that external drive.

Here is an Apple article with instructions for installing macOS to an external drive:

Install macOS on an external storage device and use it as a startup disk - Apple Support

Edit: At what stage does the installation fail? And what error message do you see when it fails?

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Answer 2

Steve Cohen Author

Level 1

59 points

May 25, 2025 11:43 AM in response to HWTech

Thank you for your reply.

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Mojave On Internal HD - Catalina On External SSD

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